Reduction of false contours by chrominance modulation

ABSTRACT

A system and method for eliminating luminance false contours within rendered documents by sacrificing chrominance accuracy within printer halftones and for using the reduction of chrominance to improve the luminance correctness for any requested color. Ordinary halftone processes are modified such that small hue shifts are traded off for the ability to eliminate luminance false contours. Luminances of colored inks are created in a manner that turns on only one color spot at a time, instead of all three, within a slowly changing gray area because numerically equivalent steps in chrominance are much less visible in printed form than changes in luminance. Removal of false contours is therefore achieved by chrominance variations rather than by spatial variations in a document.

FIELD OF THE INVENTION

The invention is generally related to image processing during documentrendering activities and, more particularly, to the reduction of falsecontours seen in a rendered image by an increase of luminance stepsduring document processing whereby the halftone level of individualcolor separations are incremented resulting in small chrominance errors,yet preserving fine spatial texture.

BACKGROUND OF THE INVENTION

In today's high-tech office, documents are often created in electronicform and then printed. Tools for document creation have becomeincreasingly powerful such as advances in high-resolution laser printersand the emerging support for full color printing. One of the problemsthat arises with the current technology is that the user may besurprised to find that the printed output does not look the same as theelectronic image displayed on the workstation. Many programs try to makethe two images match, supporting a what-you-see-is-what-you-getapproach. Hardware differences, however, may make true matchingimpossible. One such difference is resolution. A laser printer may beable to print at 300 dpi while the workstation may support only about aquarter of this density. A compensating difference is that asophisticated display may support continuous gray levels and/or colors,while a print may have to use halftones. A color display will often bedriven through a 256 element color lookup table, limiting the number ofcolors which may be displayed at any one time, while a printer, usinghalftones, can produce many more. Finally, the color primaries and whitepoint of the workstation may be different from the inks and paperavailable at the printer.

In image displays, such as CRTs, the problem of low resolution in thedisplay can be reduced by use of the display's intensity level. Incomputer graphics, this technique has long been used to reduce thealiasing effects of the low resolution display. Effects such as"jaggies" in lines as they cross pixel boundaries, and "drop out" ofsmall objects which fall between the pixels, are countered by settingthe pixel intensities to intermediate levels to reflect that part of thepixel which is covered by the object and the part which is not. Perhapsthe simplest anti-aliasing technique is to compute the image at a higherresolution, and then average the pixel values to obtain the lowerresolution intensity. For example, if the image was created at fourtimes the resolution, then four by four blocks of pixels would beaveraged to obtain the display pixel value. This is, in effect, sayingthat the display pixels are non-overlapping squares with abrupt edges.Real pixels tend to be round and overlapping, with tapering edges. Moresophisticated techniques apply a digital filter to the high-resolutionimage which, in effect, convolves the image with the pixel profile.However, in the area of desktop publishing, the effects of resolutionare strongly apparent in text. Fonts available for the coarse screenresolution are large and simplistic. They cannot show the subtle effectof different font selections. Their size may make them out of proportionto the rest of the page. One may be unable to place the characters inthe same positions as on the printed page due to crowding, giving eitherfalse page layout and line breaks, or some unreadable representation ofthe page. The anti-allasing techniques of computer graphics have beenapplied to the display of text. The fine elements and nonintegral pixelwidths of character strokes become partially illuminated pixels. Thisgives the illusion of increased resolution, and permits much smallerfont sizes to be displayed. It also permits the placing of characters atnon-integral pixel positions.

The human visual system is able to distinguish a large number of colors.To cover this range of colors, display devices often provide 256 choices(8 bits) for each of the three primary colors, red, green and blue. Thisis a total of 24 bits or 2₂₄ possible colors. To simultaneously renderany or all of these colors requires a backing frame buffer with 24 bitsper pixe. To reduce cost, displays usually provide a smaller pallet ofcolors which may be selected from this larger set. This is implementedby means of a color lookup table. A 256 entry table can be accessed byan 8-bit index. Each table entry can contain a 24-bit colorspecification. Thus, a frame buffer with only 8-bits per pixel can beused by looking up each pixel value in the color table to obtain 24-bitcolor. The display can produce 2²⁴ colors, but can render only 256 ofthem at any one time.

Color lookup tables have proven useful By changing a value in a colorlookup table, one can alter the appearance of all the pixels whichreference this table entry. This is much faster than altering the pixelsindividually. Color tables can support fast color manipulation,structuring of the image, color correction, and even animation effects.The limited set of simultaneous colors, however, presents a problemconcerning which colors should be chosen for the color table entries.Scanned images and some computer generated effects contain far morecolors than the allowed 256. One must somehow pick colors that do notdeviate too far from the true colors of the image. One would also liketo reduce the false contours which arise when a smooth variation incolor is displayed as a jump between one producible color and thenext--this is just allasing in color space. One approach to this problemis to first analyze the image to determine which colors are mostrepresentative--this can be a costly process. Another alternativeapproach is to set the table to a fixed set of distributed colors, andthen superimpose halftoning techniques to give the appearance ofintermediate colors. In this approach, resolution is being traded forintensity levels of the color primaries. The halftone screen is used tochoose between the available bounding colors for each pixel Arectangular, dispersed dot screen applied to each of the primary colorcomponents is straightforward and adequate. It also allows one todisplay the image as it is drawn. An error diffusion approach has alsobeen tried which gives slightly better results, but must be applied tothe entire image and is not used with individual image components.

Since color is a three-dimensional space, halftoning is applied threetimes for each pixel to achieve the appearance of intermediate valuesfor each of the three color coordinates. The elements of the colorlookup table are partitioned along the color coordinate axis to supportthe independent halftoning of each coordinate. If the coordinate axis isred, green and blue, the 8-bit color table index may be split into threebits of red, three bits of green, and two bits of blue. This gives eightshades of red, eight shades of green, and four shades of blue, plus alltheir combinations. Blue is chosen to receive only two bits because theeye is less sensitive to blue. An alternative is to allocate six shadesof red, six shades of green, and six shades of blue. The color table isthought of as a three-dimensional array with six elements in eachdimension which gives the mapping from color coordinates to color tableindex.

There are many possible coordinate systems used in describing colorspace besides red, green and blue. One alternative is to rotate andscale the axis so that one corresponds to the luminance Y. Luminance isa measure of how bright the color appears (yellow appears brighter thanblue). The other two coordinates give the chrominance information --anapproach used in the television industry, which implements the YIQ colormode. The Y component gives the luminance, which is shown onblack-and-white sets. All three coordinates are used for colortelevision.

One color encoding standard describes the YES color model Again, Y isthe luminance, and the E and S coordinates give the chrominance. The Ecoordinate is the green to red axis, and the S coordinate is the blue toyellow axis using the red, green, and blue primaries and white point.The YES coordinates are given by:

    Y=0.253R+0.684G+0.063B

    E=0.5R-0.5G

    S=0.25R+0.25G-0.5B

where R, G, B are the red, green and blue coordinates of the color. Oneadvantage of this color model is that the E and S coordinates are veryeasy to calculate. The YES color coordinates are not visually uniform.Equivalent displacements in these coordinates generally do not appear tothe eye as equivalent changes in color. There are visually uniform colorspaces, notably the L*a*b* and L*u*v* color models. Using thesecoordinates for color table assignments and halftoning gives slightlybetter results, since the colors appear uniformly distributed, and thehalftoning is linear with respect to appearance. However, thesecoordinates are much costlier to calculate.

U.S. Pat. No. 5,278,678 to Harrington discloses a scheme for the colordisplay of images on a device with a moderate sized color table whichfirst builds halftoned binary images for each of the three colorcomponents at a multiple of the display resolution. By summing bitvalues it then determines average values for display resolution pixels.The average values are then mapped to a color table. The YES colorcoordinate system is used by the Harrington scheme where colors areseparated into luminance and chrominance components, allowing moreweight to be given to the luminance from which the eye extracts mostinformation. The scheme provides both anti-aliasing of solid areas, suchas line and text, and color halftoning to give the appearance of colorsbetween those available from the color table.

U.S. Pat. No. 5,543,820 to Edgar discloses a method for processing colorsignals representing an image including the steps of obtaining multiplelinear color signals representing the image, translating the multiplelinear color signals to a linear luminance signal and at least onelinear chrominance signal converting the linear luminance signal to anonlinear luminance signal and converting the linear chrominance signalto an non-linear chrominance signal by dividing the linear chrominancesignal by a non-linear luminance signal. The linear processing isdirected at producing improved image detail and color.

U.S. Pat. Nos. 5,543,311 and 5,517,334 to Morag et al. provides aplurality of color volume elements which together contain at least aportion of the color space and which in effect quantize the color space.An index value is also provided for each pixel in the image where eachindex value represents a particular representative color value in thesubset of the first plurality of representative color values. The imagemay then be modified by modifying the representative color values in thesubset of the first plurality of representative color values. The image,as modified, may be displayed by using the index value for each pixel.

U.S. Pat. No. 4,812,903 to Wagensonner et al. discloses a system forelectronically enhancing an image of a colored original in order toproduce a copy of that original RGB color signals are first passedthrough a gray balancing unit and then to an luminance and chrominancegenerating unit. In the generating unit, a luminance signal and a pairof chrominance signals corresponding to color image signals of an imageto be copied are formed. The luminance signal is processed to enhancesharpness of an image, and one or both of the chrominance signals arealso processed in a color saturation adjusting unit. Lookup tables areprovided for converting the adjusted chrominance signals back tocorresponding RGB color signals.

U.S. Pat. No. 4,758,885 to Sasaki et al. discloses a method ofprocessing color images wherein a high resolution image is converted sothat it can be displayed on a lower resolution color printer or thelike. An input image of full color is converted over to a L*u*v* systemwhich is based on brightness and saturation. The L*u*v* components arethen compressed, converted into CMY signals and sent to a printer. Amethod of calculating the L*u*v* components from an RGB signal is shown.

While the related art recognizes the problems involved in processingcolor images on a continuous tone monitor, the art does not provide anefficient and effective rendering of high quality color in a mannerwhich results in minimal false contours on binary marking devices thatemploy halftoning to create intermediate tones.

A classic problem in printing documents is the representation ofintermediate tones (e.g. grays) with fixed level links (e.g. black).This is typically solved by halftoning, which is the printing of aspatially dispersed pattern of solid colored spots (e.g. black andwhite) which the eye integrates into the desired intermediate levelHowever, for a fixed spot size, producing more gray levels means usingpatterns with larger spatial areas. Using too large an area results invisible halftone patterns, while too small an area gives too few graylevels and results in false contours in the image. Refer to FIG. 1a foran example of a printed document exhibiting noticeable false contours.One is forced to compromise between these two limitations, and may findit impossible to satisfactorily produce quality print requirements.

A color printer offers new dimensions which can aid with this problem.The eye is much more sensitive to luminance than to chrominance. Falsecontours will be most visible if they arise from steps in luminance. Aconcept of contour reduction was Gray Levels from an 8 Bit ColorMonitor" presented at the 1992 SPIE/IS&T Symposium on Electronic ImagingScience and Technology, Volume 1666--Human Vision, Visual Processing,and Digital Display III, but in a form which was only applied to colormonitors and only to ramps of luminance for a single color. Theteachings in Tyler et al, however, are not effective for image renderingin printed form.

Therefore, there is a need for a system which can ameliorate the qualitytradeoff between halftones and levels that exists in digital systemswith low sampling spatial frequencies. Furthermore, there is a need toimprove print quality of documents generated by such digital systems.

Therefore, the object of this invention is to provide a method ofsacrificing chrominance accuracy within printer halftones and for usingit to improve the luminance correctness for any requested color in aprinted document, thus reducing false contours typically found inprinted documents when luminance adjustments are made.

All of the references cited herein are incorporated by reference fortheir teachings.

SUMMARY OF THE INVENTION

To achieve the foregoing objects of the invention, and to overcome theshortcomings discussed above regarding false contours that are revealedduring document printing, the invention provides a method of sacrificingchrominance accuracy within printer halftones and for using thereduction of chrominance to improve the luminance correctness for anyrequested color. Ordinary halftone processes are modified such thatsmall hue shifts are traded off for the ability to eliminate luminancefalse contours. With the invention, different luminances of colored inksare created in a manner that turns on only one color spot at a time,instead of all three, within a slowly changing gray area. Numericallyequivalent steps in chrominance will be much less visible. The schemepresented then is to sacrifice chrominance accuracy in order to achievebetter luminance behavior. Removal of false contours is done bychrominance variations rather than by spatial variations. Instead of allcolor components making a transition to a new halftone level at the samethreshold value so the colors are always balanced, the invention createsluminances of colored inks in a manner that turns on only one color spotat a time, instead of all three, within a slowly changing gray areabecause numerically equivalent steps in chrominance are much lessvisible in printed form than similar changes in luminance.

A method and system is provided which accepts desired color luminancevalues, compares the desired values with lookup tables which provide thebest color output value assigned for each desired luminance value, and ameans for providing the new color output values to a printer ink outputcontroller where a document is rendered which has reduced or eliminatedfalse contours.

In applying the method of the invention: initial color values arereceived for color specifications requested by an image processingsystem;

a luminance error is determined between said initial color values andtypical luminance values produced by halftoning;

a change in luminance is determined for said initial color values whichis the difference between said typical luminance values and a value ofany next greater halftone levels;

luminance errors and said luminances are processed to determineadjustment values for said initial color values; and

new color specifications are determined as a function of the adjustmentvalues and the initial color values for the initial color values;

The new color specifications are provided to said image processingsystem to render documents displaying reduced luminance false contourswithin printed documents.

SUMMARY OF THE DRAWINGS

The preferred embodiments and other aspects of the invention will becomeapparent from the following description which is presented to illustratea preferred embodiment of the invention when read in conjunction withthe accompanying drawings which are provided for the purpose ofdescribing an embodiment of the invention and not for limiting same, inwhich:

FIG. 1A is an illustration of a printed document without reducedchrominance and chrominance adjustments as taught by this inventiondisclosure.

FIG. 1B is an illustration of a printed document with the benefit ofchrominance adjustment as taught by the invention disclosure.

FIG. 2 is a graphical illustration of typical luminances outputs versusideal luminance outputs, and the luminance error and output stepvariations found during halftoning.

FIG. 3 is a block diagram illustration of the chrominance adjustingsystem of the invention.

FIG. 4A is a graphical illustration of typical luminance (YS) behavior.

FIG. 4B is a graphical illustration of typical luminance error (YE)behavior.

FIG. 5 is a block diagram of a typical system in which the features ofthe invention would be incorporated.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION

Referring to FIGS. 1a and 1b, an example of the results of utilizing theinvention to achieve reduced chrominance is shown by comparison of aprinted document without chrominance adjustments (1A) and withchrominance adjustments (1B). FIG. 1A contains a gray wedge printed witha 51 level halftone screen. The wedge seen if FIG. 1a is made byprinting equal amount of cyan, magenta and yellow. All the colorcomponents within the image make transitions to a new halftone level atthe same threshold value so the colors are always balanced. By contrast,through application of the invention the chrominance is corrected, butcolor components do not step at the same level--first the yellow patternchanges, then the cyan and finally the magenta This means that thechrominance is slightly wrong for most of the print, but the luminancevaries in finer steps. The result is greatly reduced luminancecontouring while the chrominance errors are not noticeable, as suggestedFIG. 1B.

Assuming a color printer with cyan (C), magenta (M) and yellow (J) inkseparations, the printer may accept color specifications to highaccuracy (eight or more bits per separation) but the actual halftonesused may be unable to produce this many levels (the halftone having only60 or 70 levels). Referring to FIG. 2, a graphical illustration of theideal versus typical luminance outputs curves is illustrated. Thetypical output is a result of halftoning measures that are used in theart to compensate for luminance errors. Also depicted is a desired coloroutput (Point A) requested in a typical system as Yin which results is alumination value between points B and C. In the prior art, the actualoutput (Yout) would be either point B or C depending on which point isclosest to the desired level. Typically, when a color value is specified(Yin), the closest level (point B or C) is received which the halftonecan produce as output (Yout). However, there will be an error (YE)between the luminance desired (point A) by the specification and theluminance actually produced by the halftones (point B or C). The changein luminance (YS) can also be determined (the difference between pointsB and C) which would occur if the next greater halftone level wereselected. As a first-order approximation one can disregard theinteraction between color separations and assume we can treat eachseparation independently. Knowing the halftone properties of the printerwe can construct tables which give the luminance error YE[i] andluminance step YS[i] for each color separation value specification i.Separate versions of these tables must be built for each of the cyan,magenta and yellow separations--YE_(C), YS_(C), YE_(M), YS_(M), YE_(J),YS_(J). For a given color specification we can use a table to find thetotal luminance error as follows:

    Ytotal=YE.sub.C [C]+YE.sub.M [M]+YE.sub.J [J]

Now, given the total luminance error and the luminance changes thatwould occur if each of the color separations were printed using the nexthalftone level (YS_(C) [C], YS_(M) [M], YS_(J) [J]) the system candecide whether or not to increment one or more of the color separationsto the next level to get a better luminance match. The simplest way tocarry this out is to construct a decision table which will be accessedby an index created by concentrating the needed information (Ytotal,YS_(C) [C], YS_(M) [M], YS_(J) [J]). This lookup table should containthe increments to the intensities of each separation needed to give thebest luminance match. It can be written as separate tables for eachseparation (D_(C), D_(M), D_(J)). The construction of such table is wellknown in the art as pointed out by the references cited herein andincorporated by reference for their teachings. The invention stepscolors alternatively, not all together, resulting in intermediate stepsin luminance through slight adjustments in chrominance.

Referring to FIG. 3, the system 1 provided to accomplish false contourreduction by the alternate chrominance adjustment method of theinvention is graphically summarized. Desired color values are providedthrough inputs 2A-2C at the left side of FIG. 3. While colors can beexpressed in a variety of different color coordinate systems, thepreferred embodiment specifies the color in the coordinate used forhalftoning. This typically is the specification of ink or toner amount,cyan (C), magenta (M), yellow (J) and perhaps black (K). It should bepointed out that black (K) can also be considered in the invention;however, this disclosure is limited to cyan, magenta and yellow forsimplicity of description. The ideal colorant amount can be specified tofine precision. For each colorant specification there is an ideal amountof luminance absorption. This is the reduction of luminance that wouldbe caused by the colorant if that ideal level of colorant amount couldbe placed on the medium. Thus, the presence of the halftone causes anerror between the ideal and actual luminance absorption for a specifiedcolorant amount. The error is zero when the specification matches one ofthe levels that the halftone can produce, but grows smoothly as thecolorant amount increases until the next halftone level is reached. FIG.4A illustrate typical halftone levels that may be produced for a desiredluminance YS. Thus, referring to FIG. 4B, the error function, which isrelated to the outputs desired luminances in FIG. 4a, has an overallsawtooth shape. It is these error functions, determined for the C, M, Jcolorants, that are saved into tables YE_(C), YE_(M), and YE_(J)respectively, as shown in FIG. 3. Using the colorant amounts to indexthese tables yields the luminance errors due to halftoning made by eachof the colorants. The total luminance error is approximated as the sumof the errors from the components by routing the component error to theadder 3.

Halftones quantize the producible colorant levels to a limited set ofvalues. For any ideal colorant amount one can determine the closestproducible levels. The difference in the bounding levels is what isdesired--that is, the luminance difference between the halftone levelbelow the idea luminance amount and the halftone level above the ideallevel. This difference can be stored in tables YS_(C), YS_(M), andYS_(J), and indexed by the ideal amount. These tables provide the amountby which the luminance would change if one were to employ the upperhalftone level instead of the lower halftone value for approximating thedesired color.

It has been described how one can determine the total luminance errorthat would occur if the lower halftone levels are employed, and theadjustments to the luminance possible by changing the halftone level ineach of the colorants. One can use this information to decide whichhalftone levels to select to minimize the luminance error. Note thatthese values are small since they are changes in luminance and notabsolute luminance values. Since they are small numbers only a few bitsare needed to represent them. This means that the values can beconcatenated to form an index into a table of reasonable size. Forexample, if one were to allocate two bits for the step size in yellowand two bits for cyan, three bits for magenta and four bits for thetotal luminance error, then their concatenation gives an eleven-bitindex that corresponds to a 2048 element table. Such a table couldcapture the logic for selection of the optimal halftone levels for agiven luminance error and set of step sizes. Note that alternative bitallocations can be used. Also note that the scale of luminance changescorresponding to the index bits need not be the same for each colorant.A two-bit code for yellow need not cover the same luminance range as atwo-bit code for cyan. Adjustments for luminance range can be includedin the decisions that are used in selecting the best halftone levels.

Referring again to FIG. 3, the decision processor 4 used for convertingthe information about the luminance error and step sizes into halftonelevel specification results in increments encapsulated in tables D_(C),D_(M) and D_(J), which provide the possible increments for the finalcolorant specification values. The invention combines the possibleincrements with the original color specification at the adders 5A-5C.Depending on their index, the D tables either provide zero which givesthe original halftone level or an increment which will step the colorantspecification to the next halftone level. This method of selectinghalftone levels works best when the halftone thresholds are uniformlyspaced. If the halftone levels are nonuniform, then some variation inthe method should be employed such as using tables to map values intoand/or out of a uniformly varying coordinate.

New chrominance adjustment values from tables D_(C), D_(M), and D_(J)are summed with original chrominance values C, M, and J from systeminputs 2A-2C resulting in final values C', M', and J'. The final valuesare provided at outputs 6A-6C to a document rendering system such as anink printer. The new ink inputs received by the printer are colorseparations, C', M' and J', which will result in the substantialreduction or elimination of false contours.

Referring to FIG. 5, a block diagram of a typical system in which theinvention would be implemented is illustrated. The typical system wouldhave a document creation means 11 or a document scanner 12. A documentfrom either means of document generation must be processed by aprocessor 13. Typical PDL interpretation and raster imaging processingwill occur depending on the origin of the document. The document is thenprocessed through a frame buffer 14 and then through a halftoner 15. Itis through luminance processor 16 operations which manipulate the framebuffer 14 and halftoner 15 that luminance and chrominance adjustmentstaught herein can be applied to the document, resulting in reducedhalftoning. The adjusted document may then be rendered at a printer 17or marking devices well known in the art.

The Advantage of the invention is the reduction of false contours in arendered image by incrementing the halftone level of individual colorseparations resulting is small chrominance errors, yet preserving finespatial texture of the resulting image.

While the invention is described with reference to a particularembodiment, this particular embodiment is intended to be illustrative,not limiting. Various modifications may be made without departing fromthe spirit and scope of the invention as defined in the amended claims.Modifications and alterations will occur to others upon reading andunderstanding this specification; therefore, it is intended that allsuch modifications and alterations are included insofar as they comewithin the scope of the appended claims or equivalents thereof.

I claim:
 1. A method of reducing luminance false contours within printeddocuments by sacrificing chrominance accuracy within printed halftones,comprising:receiving initial color values for color specificationsrequested by an image processing system; determining a luminance errorbetween said initial color values and typical luminance values producedby halftoning; determining a change in luminance for said initial colorvalues which is the difference between said typical luminance values anda value of any next greater halftone levels; processing said luminanceerrors and said luminances to determine adjustment values for saidinitial color values; and determining new color specifications as afunction of said adjustment values and said initial color values forsaid initial color values; wherein said new color specifications areprovided to said image processing system to render documents displayingreduced luminance false contours within printed documents.
 2. The methodof claim 1 wherein said new color specifications comprise colorseparations that are stepped alternatively resulting in intermediatesteps in luminance through slight adjustments in chrominance.
 3. Themethod of claim 2 wherein said color specifications are for cyan (c),magenta (m) and yellow (j).
 4. The method of claim 3 wherein said colorspecifications includes black (k).
 5. The invention of claim 1 wherein adetermination of whether to increment one or more of said color valuesto an increased level is made given a comparison of said luminance errorand said luminance changes that would occur if each of said colorseparations were rendered using the next higher halftone level andwherein said image processing system will decide whether to incrementone or more of said color separations to the next higher level to get abetter luminance match based on information stored in said memoryrelating to possible increments to said specified colors intensities foreach separation needed to provide a best luminance match.
 6. The methodof claim 5 wherein said steps of determining error and determiningluminance steps is implemented by utilizing lookup tables indexed byconcatenated values of total luminance error and luminance changes withhalftone levels for said color separations.
 7. A system for reducingluminance false contours within a printed document, comprising:receivingmeans for receiving initial color values for color separationspecifications requested by a image processing system; error determiningmeans for determining the error between said specified color values andtypical luminance values produced by halftoning; luminance stepdetermining means for determining the luminance steps for said initialcolor values which is the difference between said typical luminancevalues and the value of the next greater halftone levels; processingmeans for processing said luminance errors and said luminance steps todetermine adjustment values for said initial color values; new colorspecification means for receiving said adjustment values and saidinitial color values and for providing new color specifications for saidinitial color values based on a combination of said adjustment valuesand said initial color values; and means for providing said new colorspecifications to said image processing system wherein said imageprocessing system will render documents displaying reduced luminancefalse contours within printed documents.